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Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penr

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means. Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters. A lot! Fundamentally, relativity does not allow observers to disagree on observations of what, only when. If an outside observer never observes someone falling into a black hole then they cannot fall in. It's just that simple! Again, for the slow people at the back of the class: Observers cannot disagree on this. If there is a paradox in your model, then your model is broken, end of story. There is this weird aspect of modern physics of holding on to the almost-mystical "woo" aspects far more than is justified in either the mathematics or observations. Quantum Mechanics and cosmology are especially rife with popularisations of what amounts to science fiction story telling. It's fun to think about wormholes, white holes, alternate universe accessible trough black holes, etc... I've read these novels, they were fun! Not physical though, natch. Much more realistically: An infalling observer is slowed down relative to the outside universe and the stellar remnant that formed the black hole. Effectively, the hapless infalling victim sees the black hole and the universe both "speed up". The error most people make here is that they assume that the black hole is already fully formed in the infinite past and is "completed", making it a mathematically perfect sphere in a sense. No! It hasn't finished forming because as its gravitational field increases its time distortion increases. Its formation is "frozen in time" (actually just very very slow), it is never fully formed. Infalling observers see this slowdown speeding back up, so what they observe is the light of the black hole evaporation getting blue-shifted and brighter until right before mathematically they would have "fallen past the event horizon" what they're actually seeing is akin to a supernova exploding in their face. They're blasted into subatomic particles, joining the radiation of the black hole evaporation many trillions of years into the future... but not infinitely into the future. This model solves every paradox of black holes, making them boring and not sci-fi exciting again, which is why you haven't heard about it! Uncool stories don't get repeated on blogs for ad-clicks. Just remember: there are no infinities in the universe, and anyone using one in their theory is almost certainly just making stuff up to sound cool. PS: Penrose is also behind the kook notion that brains are quantum despite all evidence.

while_true_ 2y ago
It irks me so many physicists/cosmologists jump from the mathematical GR singularity at the center of a BH to "matter there has infinite density." That's highly unlikely, it's probably quark plasma.
- jiggawatts 2y ago
  My simple model of it is to just think about spatial surfaces and do "accounting" of the total flux through them. If you draw a sphere around a star collapsing into a black hole, you can treat it as a closed system. If the black hole evaporates, then all of the mass-energy of its progenitor original star needs to leave through these concentric surfaces. This is on the same order of magnitude as a supernova, as it is equal to the collapsed core of the star being converted into pure radiated energy! An infalling observer accounting of the mass-energy flows must match this external view point. As they cross smaller and smaller bounding spheres, they must see this energy flowing out through those boundaries, adding up to the same total. (There is nowhere else for the energy to go; it has to be blasting you in the face as you fall in!) Oversimplified models of black holes concentrate the mass-energy to a point, leaving spacetime around it an empty vacuum. So an infalling observer will see zero, zero, zero, zero... infinite energy density for an infinitesimal time. This is non-physical nonsense, and isn't even mathematically sound! From Hawking we know that infalling (and distant) observers see some finite energy flux, and from Einstein's GR we know that infalling observers will see this blue-shifted and time-accelerated on the way in. The logical conclusion is that the flux is observed to increase smoothly by infalling observers until the entire amount is accounted for in a finite time. This is a staggering total amount of radiated energy, equivalent to an matter/anti-matter explosion of matter the density of a neutron star core! There is no way anything could "fall through" this while jotting down their observations. It's not a survivable journey. There are no wormholes, reachable parallel universes, and there are no separate white holes "elsewhere" in the universe. A black hole is the white hole, smeared out trillions of years into the future so that the enormous total energy is radiated out so slowly from an outside perspective that they just look black. Infalling observers see the "true" white nature of these explosions frozen in time.
  - 2y ago

while_true_ 2y ago

It irks me so many physicists/cosmologists jump from the mathematical GR singularity at the center of a BH to "matter there has infinite density." That's highly unlikely, it's probably quark plasma.

jiggawatts 2y ago
My simple model of it is to just think about spatial surfaces and do "accounting" of the total flux through them. If you draw a sphere around a star collapsing into a black hole, you can treat it as a closed system. If the black hole evaporates, then all of the mass-energy of its progenitor original star needs to leave through these concentric surfaces. This is on the same order of magnitude as a supernova, as it is equal to the collapsed core of the star being converted into pure radiated energy! An infalling observer accounting of the mass-energy flows must match this external view point. As they cross smaller and smaller bounding spheres, they must see this energy flowing out through those boundaries, adding up to the same total. (There is nowhere else for the energy to go; it has to be blasting you in the face as you fall in!) Oversimplified models of black holes concentrate the mass-energy to a point, leaving spacetime around it an empty vacuum. So an infalling observer will see zero, zero, zero, zero... infinite energy density for an infinitesimal time. This is non-physical nonsense, and isn't even mathematically sound! From Hawking we know that infalling (and distant) observers see some finite energy flux, and from Einstein's GR we know that infalling observers will see this blue-shifted and time-accelerated on the way in. The logical conclusion is that the flux is observed to increase smoothly by infalling observers until the entire amount is accounted for in a finite time. This is a staggering total amount of radiated energy, equivalent to an matter/anti-matter explosion of matter the density of a neutron star core! There is no way anything could "fall through" this while jotting down their observations. It's not a survivable journey. There are no wormholes, reachable parallel universes, and there are no separate white holes "elsewhere" in the universe. A black hole is the white hole, smeared out trillions of years into the future so that the enormous total energy is radiated out so slowly from an outside perspective that they just look black. Infalling observers see the "true" white nature of these explosions frozen in time.
- 2y ago